1. Field of the Invention
The present invention relates in general to an image display method and an image display unit, and in particular to an improvement in the output brightness characteristic between an input signal and its output brightness.
2. Description of the Related Art
In the medical field, a human body is irradiated with radiation, the radiation-transmitted image is recorded on X-ray film, and while the X-ray film recorded with radiation-transmitted image is being held to a light source or schaukasten (which is a box for observing X-ray film), the transmitted light image is observed and analyzed.
On the other hand, because of the recent development of digital technology it is becoming standard to digitize and process the aforementioned radiation-transmitted image by computers. It has also become possible to observe and analyze the aforementioned radiation-transmitted image stored as a digital image on a server, etc., by displaying it immediately on an image display unit, such as a cathode-ray tube (CRT) display unit, connected to a network, without outputting it to X-ray film for every display.
If an image as viewed with the X-ray film held to the schaukasten is compared with an image as viewed with it displayed on a CRT display unit, etc., incidentally, there is a difference in how the two images look.
The image display unit, such as a CRT display unit, etc., is generally grouped into two types. One type has an output brightness characteristic in which an input image signal S and its output brightness L are in a linear relationship (FIG. 3A). Another type has an output brightness characteristic in which an input image signal S and a logarithmic value Y (=log10(L)) of its output brightness L are in a linear relationship (FIG. 3B). The display unit having the output brightness characteristic shown in FIG. 3B is most suitable to make the human eyes feel contrast (hereinafter referred to as visual effect).
On the other hand, the X-ray film, as shown in FIG. 4A, exhibits an output brightness characteristic in which an input image signal S and its output density D are approximately linear, but the sensitivity of the output density D with respect to the input image signal S is reduced at the low signal value region. Also, when viewing an image recorded on X-ray film of such an output brightness characteristic, with the film held to the schaukasten, an image portion of high density is recognized as an image portion of low brightness and an image portion of low density is recognized as an image portion of high density, as shown in FIG. 4B. Therefore, for the image viewed with the X-ray film held to the schaukasten, the sensitivity of the output brightness (logarithmic value) to the value of an input signal becomes lower in the low signal value region corresponding to the image portion of low density than in other intermediate and high signal value regions.
Thus, between an image as viewed with X-ray film held to the schaukasten and an image as viewed with it displayed on an image display unit such as a CRT display unit, etc., there is a difference in the sensitivity of the output brightness in the low signal value region. Because of this, there are cases where doctors, etc., who are used to the analysis of an image output to X-ray film which is the traditional method of analyzing an image, will feel a sense of incompatibility with respect to an image displayed on an image display unit. There is also a possibility that a sense of incompatibility such as this will inhibit disturb accurate analysis.